1. Field of the Invention
The present invention relates to a power transmission shaft for transmitting running torque.
2. Description of the Related Art
In a drive shaft of an automobile or the like, a power transmission shaft is coupled to a constant velocity universal joint. In many cases, the power transmission shaft is coupled to an inner ring of the constant velocity universal joint with splines or serrations (hereinafter collectively represented by the “splines”) formed at a shaft end. To mold the splines of this type of power transmission shaft, plastic working such as form rolling and press working is used in consideration of workability and cost.
There are various types of shape of an end portion of the spline, which is formed on the power transmission shaft, on the opposite side of the shaft end. There are a type, for example, in which a valley section 12a of a tooth 12 on the side of a power transmission shaft 10 directly leads to an outer surface of the power transmission shaft 10 as shown in FIG. 6 (hereinafter called “direct incision type”), a type in which the valley section 12a of the tooth 12 leads to the outer surface of the power transmission shaft 10 with smoothly enlarging the diameter of the valley section 12a as shown in FIG. 7 (hereinafter called “round up type”) and the like. In the “round up type,” a surface of the valley section 12a, the diameter of which is smoothly enlarged, (hereinafter called “diameter expansion surface”) is generally formed in the shape of an arc with a radius of Ra in cross section (there is a case that the surface is formed in the shape of a spherical surface). In this case, it is known that increase in the radius of curvature Ra of the diameter expansion surface can increase the effect of relieving stress, and hence improve the strength of the power transmission shaft.
Japanese Patent Laid-Open Publication No. 2000-97244 discloses an example of the power transmission shaft having the foregoing round up type of splines. In this power transmission shaft, a diameter expansion area is formed at an end of the spline, and a section engaging with an corresponding outer member is formed inside the diameter expansion area. Accordingly, it is possible to improve the strength of the power transmission shaft in terms of both static strength and fatigue strength.
By the way, in recent years, the enhancement of emission control, the improvement of fuel efficiency and the like are strongly desired in the automobile due to an upsurge of interest in environmental issues. As part of measures against them, further weight reduction and improvement in strength are strongly desired in the power transmission shaft such as a drive shaft and a propeller shaft.